Photoimageable compositions useful as resists for forming printed circuits, printing plate, solder masks or the like have been used for some time now. The earliest photoresists were solvent-borne and solvent-developable. The development of aqueous-developable resists represented an advance in reducing solvent emissions in the immediate workplace and in the general environment. The continued emphasis in reducing organic solvent both from the workplace and from the general environment has spurred the search for waterborne photoresists which are formulated and applied as aqueous liquids.
U.S. Pat. No. 5,045,435, the teachings of which are incorporated herein by reference, describes a waterborne photoresist composition which is developable in alkaline aqueous solution. The composition comprises a multifunctional monomer, a photoinitiator and a latex of water-insoluble carboxylated acrylic copolymer. To stabilize the composition and to adjust the viscosity, this patent teaches neutralizing the latex polymer to at least 25% with a base, such as ammonia, another amine, or sodium hydroxide.
A liquid, such as a waterborne photoresist liquid will only wet and spread over a surface if the forces of attraction between the liquid and the substrate are greater than the cohesive forces within the liquid. Accordingly, the surface energy of the liquid must be at or below the surface energy of the substrate. Thus, to coat a copper substrate having a surface energy of 35 dynes/cm, the surface energy of water, which has a surface energy of 72 dynes/cm, must be dropped. This may be accomplished with the aid of a suitable surfactant. A surfactant is a material with both hydrophilic and hydrophobic properties. Surfactants can be divided into three major chemical classes: hydrocarbons, silicones, and fluorochemicals, these designations referring to the hydrophobic portion of the molecule. The hydrophobe is attached to a hydrophilic group which can be anionic, cationic or nonionic in nature.
Applicants have found neither hydrocarbon surfactants nor silicon surfactants to be particularly suitable surfactants in waterborne photoimageable compositions. Although all types of surfactants are found to sufficiently lower surface energy, it is found that hydrocarbon and silicone surfactants encounter problems respect to dewetting of the copper substrate. Dewetting produces voids which result in an unsuitable coating. Furthermore, if the photoimageable composition is intended as a plating resist, silicone from a silicone-based surfactant can leach into the plating solution creating potential bath failure. Of the major classes of surfactants, hydrocarbon based surfactants are found to be the least effective in reducing the surface tension of the liquid and do not produce defect coatings. Furthermore, hydrocarbon based surfactants contribute to the overall volatile organic components (VOCs) in the coating, which is contrary to the purpose of producing waterborne coatings.
Above-referenced U.S. Pat. No. 5,045,435 gives examples wherein fluorocarbon-based surfactants are used in waterborne photoimageable compositions, i.e., those sold under the trademark FLUORAD FC-431 and FC-120. Applicants have found that neither of these performs well. FC-120, which is anionic, wets sufficiently, but produces increased foaming. FC-431 is non-ionic, but is found to be less than suitable in that it does not eliminate all coating defects, regardless of the concentration level.
It is a general object of the present invention to provide a surfactant for waterborne photoimageable compositions which lowers the surface tension sufficiently for coating on a substrate, such as a copper substrates, does not dewet, and does not cause foaming.